Optical fiber gratings have many applications and are widely used in fiber optic communication systems, fiber optic sensors and fiber lasers to selectively control the wavelength of light propagating in an optical fiber. A typical fiber grating includes a length of optical fiber in which a section of the fiber core has been modified to include a plurality of periodic perturbations in refractive index along the length of the fiber. Generally, there are two types of fiber gratings that are formed in this manner: Fiber Bragg Gratings (FBGs) and Long Period Fiber Gratings (LPFGs). LPFGs are distinguished from FBGs by differences in the periodic spacing of the perturbations.
FBGs reflect light at a wavelength λB, characterized by λB=2nΛB, known as the Bragg condition, or Bragg wavelength, where λB is the center wavelength of reflected light from the grating, n is the effective refractive index of the fiber core, and ΛB is the period of refractive index modulation in the fiber. FBGs generally have good wavelength selection capability as a narrow band reflective mirror. The center wavelength, a.k.a., resonance wavelength, of an FBG may be affected by changes in strain and temperature. For example, for a given strain εz, the center wavelength shift of the FBG is ΔλB=λB(1−p)εz, where p is an effective strain-optic constant. For a given temperature change ΔT, the center wavelength shift is ΔλB=λB(αA+αB)ΔT, where αA is the thermal expansion coefficient of the fiber and αB represents the thermo-optic coefficient. For a typical FBG with center wavelength at 1550 nm, the strain induced wavelength shift is about 2 pm/με, and the temperature change induced wavelength shift is around 12.8 pm/° C. These physical characteristics can be used to tune the center wavelength of a FBG, i.e., by applying controlled strain or heat to the FBG.
LPFGs have a physical configuration similar to that of FBGs, but the LPFG grating period ΛL is much longer than the FBG grating period ΛB. In particular, ΛL is typically 200˜2000 times longer than ΛB. The LPFG operates by coupling the fundamental mode in the fiber core to the cladding modes of the fiber. The excited cladding modes are then attenuated, resulting in the appearance of resonance loss in the transmission spectrum. Consequently, in contrast to FBGs, LPFGs do not produce reflected light. Phase matching between the fundamental mode and cladding modes at wavelength λmL can be expressed as: λmL=(ncore−nclm) ΛL; where, ncore is the effective refractive index of the fundamental mode and nclm is the effective refractive index of the mth cladding mode, and ΛL is the period of the LPFG. Since several cladding modes can satisfy this condition, each one is at different center wavelength λmL. Consequently, the transmission spectrum of the LPFG exhibits a series of transmission loss peaks along the spectrum distribution. Similar to FBGs, the center wavelength (resonance wavelength) of LPFGs is also affected by changes in strain and temperature. Therefore, the resonance wavelength of LPFG can be tuned by applying controlled strain or heat to the LPFG.
For applications including but not limited to fiber grating-based tunable filters, fiber sensor demodulation systems and tunable fiber lasers, it is desirable to be able to tune the resonance wavelength of fiber gratings over a large wavelength range. As already mentioned, it is known to tune a fiber grating via strain, e.g., stretching or compressing a fiber grating, and also via application of heat, e.g., directly heating the fiber grating or using a heating element packaged with the fiber grating to apply a strain on fiber grating. However, thermal tuning is somewhat problematic because it can cause degradation of the fiber grating, and the tuning range is relatively small due to the practical limits of the temperatures that can be applied. With regard to strain tuning, it is known that compressing a fiber grating provides a potentially greater tuning range than stretching the fiber grating because an optical fiber is up to 20 times stronger in compression than in tension. However, since the fiber is very thin, e.g., a typical diameter of about 125 um, applying axial compression strain to the fiber without inducing buckling of the fiber presents some difficulty.
Techniques are known for preventing compression buckling. One technique, described by Morey, et al in U.S. Pat. No. 5,469,520, entitled “Compression Tuned Fiber Grating,” is to put a FBG in sliding ferrules and place the ferrules in a mechanical structure to guide and confine the fiber. However, the Morey's technique requires ferrules of precise diameter, and highly accurate ferrule alignment. Another technique, described by Fernald et al in U.S. Pat. Nos. 6,229,827 and 6,363,089 entitled “Compression-Tuned Bragg Grating and Laser,” fuses the FBG in a glass capillary tube. However, the resulting device is difficult to handle during manufacturing operations. Another technique, described by Long in U.S. Pat. No. 6,360,042, entitled “Tunable optical fiber gratings device,” is to bond the FBG on a cantilever beam. The beam can then be bent in different directions, resulting in application of compressive or tensile strain of the FBG. It would be desirable to have an improved technique to facilitate tuning of fiber gratings over a wide wavelength range that does not suffer some or all of the limitations of known techniques.